Geological Society of Australia Special Publication 22, 377–397

CHAPTER 25—120 to 0 Ma tectonic evolution of the southwest Pacific and analogous geological evolution of the 600 to 220 Ma Tasman Fold Belt System

A. J. CRAWFORD1*, S. MEFFRE1 AND P. A. SYMONDS2

1 Centre for Ore Deposit Research, University of Tasmania, Hobart, Tas 7005, Australia. 2 Geoscience Australia, GPO Box 378, Canberra, ACT 2600, Australia. * Corresponding author: [email protected]

We review the tectonic evolution of the SW Pacific east of Australia from ca 120 Ma until the present. A key factor that developed early in this interval and played a major role in the subsequent geodynamic history of this region was the calving off from eastern Australia of several elongate microcontinental rib- bons, including the Lord Howe Rise and Norfolk–New Caledonia Ridge. These microcontinental ribbons were isolated from Australia and from each other during a protracted extension episode from ca 120 to 52 Ma, with oceanic crust accretion occurring from 85 to 52 Ma and producing the Tasman Sea and the South Loyalty Basin. Generation of these microcontinental ribbons and intervening basins was assisted by emplacement of a major mantle plume at 100 Ma beneath the southern part of the Lord Howe Rise, which in turn contributed to rapid and efficient eastward trench rollback. A major change in Pacific plate motion at ca 55 Ma initiated east-directed subduction along the recently extinct spread- ing centre in the South Loyalty Basin, generating boninitic lithosphere along probably more than 1000 km of plate boundary in this region, and growth of the Loyalty–D’Entrecasteaux arc. Continued sub- duction of South Loyalty Basin crust led to the arrival at about 38 Ma of the 70–60 million years old west- ern volcanic passive margin of the Norfolk Ridge at the trench, and west-directed emplacement of the New Caledonia ophiolite. Lowermost allochthons of this ophiolite are Maastrichtian and Paleocene rift tholeiites derived from the underthrusting passive margin. Higher allochthonous sheets include a poorly exposed boninitic lava slice, which itself was overridden by the massive ultramafic sheets that cover large parts of New Caledonia and are derived from the colliding forearc of the Loyalty–D’Entrecasteaux arc. Post-collisional extensional tectonism exhumed the underthrust passive margin, parts of which have blueschist and eclogite facies metamorphic assemblages. Following locking of this subduction zone at 38–34 Ma, subduction jumped eastward, to form a new west-dipping subduction zone above which formed the Vitiaz arc, that contained elements which today are located in the Tongan, Fijian, Vanuatu and Solomons arcs. Several episodes of arc splitting fragmented the Vitiaz arc and produced first the South Fiji Basin (31–25 Ma) and later (10 Ma to present) the North Fiji Basin. Collision of the Ontong Java Plateau, a large igneous province, with the Solomons section of the Vitiaz arc resulted in a reversal of subduction polarity, and growth of the Vanuatu arc on clockwise-rotating, older Vitiaz arc and South Fiji Basin crust. Continued rollback of the trench fronting the Tongan arc since 6 Ma has split this arc and produced the Lau Basin–Havre Trough. This southwest Pacific style of crustal growth above a rolling-back slab is applied to the 600–220 Ma tectonic development of the Tasman Fold Belt System in southeastern Australia, and explains key aspects of the geological evolution of eastern Australia. In particular, collision between a plume-trig- gered 600 Ma volcanic passive margin and a 510–515 Ma boninitic forearc of an intra-oceanic arc had the same relative orientation and geological effects as that which produced New Caledonia. A new subduction system formed probably at least several hundred kilometres east of the collision zone and produced the Macquarie arc, in which the oldest lavas were erupted ca 480 Ma. Continued slab roll- back induced regional extension and the growth of narrow linear troughs in the Macquarie arc, which persisted until terminal deformation of this fold belt in the late-Middle to Late Devonian. A similar pattern of tectonic development generated the New England Fold Belt between the Late Devonian and Late Triassic. Parts of the New England Fold Belt have been broken from Australia and moved oceanward to locations in New Zealand, and on the Lord Howe Rise and Norfolk–New Caledonia Rise, during the post- 120 Ma breakup. Given that the Tasman Fold Belt System grew between 600 and 220 Ma by crustal accretion like the southwest Pacific since 120 Ma, facing the open Pacific Ocean, we question whether the eastern (Australia–Antarctica) part of the Neoproterozoic Rodinian supercontinent was joined to Laurentia. KEY WORDS: Australia, Lachlan Fold Belt, southwest Pacific, subduction, Tasman Fold Belt, tectonics.

INTRODUCTION north–south-oriented fold belts (Figure 1) that record a con- tinuing history of crustal growth from 600 Ma until at least The Tasman Fold Belt System makes up most of the crust the Triassic. Taking into account the 800–600 Ma Adelaide of eastern Australia and consists of three broadly Rift Complex further west, this eastward younging of the 378 A. J. Crawford, S. Meffre and P. A. Symonds

Figure 1 Generalised map of the Tasman Fold belt System and Adelaide Rift Complex showing constituent fold belts (modified from Scheibner 1996).

fold belts has been ascribed by several generations of Phanerozoic, of fold belts that display all or most of the Australian geologists to the progressive accretion to the stratotectonic features of these collisional accretion fold Australian Mesoproterozoic nucleus of outboard elements belts, it is clear that they have been important in the growth along the southwestern margin of the remarkably long-lived of continental crust. Pacific Ocean (Crook 1980). There is consensus that conti- Interpreting the eastern Australian fold belts in an actu- nent–continent collisions have not been involved in gener- alistic framework relies heavily on an understanding of the ating eastern Australian continental crust (Coney 1992; temporal evolution of modern western Pacific-type subduc- Veevers 2000a; Collins in press). tion-related settings. Implicit in this ‘the present is the key The Tasman Fold Belt System formed by collisions of to the past’ framework is that the processes and products of arcs or microcontinents with continents, a process the modern western Pacific arc–backarc basin systems (and Scheibner (1996 p. 21) referred to as ‘collisional accretion’. their relative dispositions?) are akin to those operating at Such a mechanism of continental growth is not restricted to least as far back as 600–500 Ma around the proto-Pacific. the present western Pacific margin. A broadly similar tec- Therefore, a broad-ranging knowledge of modern western tonic style is recorded from the Altaids of Asia (Sengor & Pacific geodynamics and the tectonic settings of magma Natal’in 1996), the Pan-African fold belts of the Arabian genesis is a prerequisite to successfully unravelling the geo- Shield (Johnson et al. 1987), and the Cordilleran mountain logical evolution of the Neoproterozoic–Palaeozoic fold belt of western North America (Coney 1987). Given the belts of eastern Australia. In fact, it could be convincingly global distribution, since at least the start of the argued that the geodynamic evolution of the eastern margin Tectonic evolution: SW Pacific 379 of the Australian plate since ca 120 Ma, including the open- transport of crustal elements originally contiguous with ing of several major and minor ocean basins, and formation eastern Australia (including growth of at least two marginal of several microcontinents and intra-oceanic arc systems, is basins floored by oceanic crust), it is difficult to conceive simply a continuation of the same regime of tectono-mag- the 90–45 Ma plate boundary east of Australia being any- matic events that formed the continental crust of Australia thing but a convergent boundary. over the preceding 500 million years. In this paper, we first examine the post-120 Ma geody- Tasman Sea–New Caledonia Basin–South Loyalty namic and tectono-magmatic evolution of the eastern mar- Basin opening gin of the Australian plate between the Tonga–Kermadec– Fiji–Vanuatu–Solomons arc systems and the eastern GEOPHYSICAL RECORD Australian continental margin. Armed with this informa- tion, we then investigate the geological evolution of south- The New Caledonia Basin lies east of the Lord Howe Rise eastern Australia since 600 Ma, and identify key and west of the Norfolk–New Caledonia Ridge (Figure 2). components of former plate-margin settings now incorpo- Most studies suggest that extension in the New Caledonia rated into the continental crust of this region. A comparison Basin commenced in the Early Cretaceous and that this of the active (and recently inactive) in situ crustal elements basin is underlain by extended continental lithosphere of the western Pacific east of Australia with the cratonised (Etheridge et al. 1989; Uruski & Wood 1991; Sdrolias et al. crust of southeastern Australia lends insight into the 2002), although Sutherland (1999) suggested that the processes that generate continental crust, and into crustal southern part may have a limited amount of oceanic-type growth western Pacific-style. crust. Sdrolias et al. (2002) have proposed that the New Caledonia Basin formed as a backarc rift behind the Norfolk Ridge volcanic arc above a west-dipping Benioff FRAGMENTATION OF CONTINENTAL CRUST zone involving subduction of Cretaceous (before 120–100 OVER A RETREATING SLAB Ma) Pacific Ocean crust. Ocean crust accretion in the Tasman Sea commenced Eastern margin of the Australian Plate 120–45 Ma around 84 Ma southeast of Tasmania, and propagated northward; by 64 Ma the Coral Sea commenced opening The nature of the eastern boundary of the Australian Plate (Gaina et al. 1998). All spreading between the Lord Howe between 120 and 45 Ma is poorly understood. However, Rise (and associated crustal elements) and Australia had there is abundant evidence that commencing around ceased by 52 Ma. 130–120 Ma, the continental crust of the eastern part of Oceanic crust lying between the New Gondwana passed into a long-lived episode of extension Caledonia–Norfolk Ridge and the east-dipping modern and eventual rifting. This led to fragmentation of continen- subduction zone of the Vanuatu–Solomons arc systems has tal crust broadly parallel with the present eastern coast of been referred to as the South Loyalty Basin for that part Australia, to form three subparallel marginal basins—from west of the Loyalty–D’Entrecasteaux Ridge, and the North west to east, the Tasman Sea, the New Caledonia Basin and Loyalty Basin between that ridge and the Vanuatu (New the Loyalty Basin (Symonds et al. 1996; Auzende et al. Hebrides) island arc. A large part of the North Loyalty 2000; Müller et al. 2000)—and two elongate microconti- Basin has been subducted beneath the Vanuatu arc, at nental ribbons, the Lord Howe Rise and the New least over the last 5–10 million years during opening of the Caledonia–Norfolk Ridge (Figure 2). The first sign of even- North Fiji Basin. tual rifting and ocean opening was the development of a Kroenke (1984) and Bitoun and Recy (1982) suggested 2500 km-long continental rift along the eastern margin of that the South Loyalty Basin formed in the Eocene. In con- Australia mainly between 120 and 95 Ma, characterised by trast, Dubois et al. (1973) and Collot et al. (1987) argued immense volumes of largely felsic magmas (Bryan et al. that the South Loyalty Basin formed as a marginal basin 1997). A southern continuation of this continental rift is prior to 80 Ma above an east-dipping subduction zone, presently unknown, but its presence along the central and with a trench west of New Caledonia, implying subduction southern Lord Howe Rise is likely given the immense vol- of oceanic crust of the New Caledonia Basin. New seismic ume of sediments derived from these rocks that was interpretations of South Loyalty Basin sediment fill from the deposited in the Bass and Otway Basins in the Early southern part of the basin away from the zone of Eocene Cretaceous. emplacement of the New Caledonia ophiolite suggest that There is insufficient evidence to be sure whether sub- the South Loyalty Basin formed by extension and later duction continued east of the developing rift systems ocean crust accretion around 120 Ma in the Early between 120 and 45 Ma. Collot et al. (1987) have invoked Cretaceous (Auzende et al. 2000), perhaps contemporane- east-dipping subduction west of New Caledonia from 80 ous with the opening of the New Caledonia Basin. Ma to Early Paleocene (ca 64 Ma), although there are Cretaceous oceanic crust in the South Loyalty Basin is also presently no known rocks of this age with subduction- demanded by the model of Sdrolias et al. (2002). related signatures to support this model. Sdrolias et al. (2002) also argued for a long period of east-directed sub- GEOLOGICAL RECORD duction east of the Lord Howe Rise during the period from around 90 Ma until about 45 Ma. Given the general north- Although the nature and age of oceanic crust in the Tasman ward (albeit slow) absolute motion of the Australian plate Sea is well demonstrated by magnetic anomalies, basement during this interval, and the major extension and eastward has only been sampled at DSDP Site 283, southeast of 380 A. J. Crawford, S. Meffre and P. A. Symonds

Figure 2 Southwest Pacific east of Australia showing gravity anomalies based on satellite altimetry and major morphotectonic ele- ments. NLB, North Loyalty Basin; LA, Loyalty arc; NR, Norfolk–New Caledonia Ridge; VA, Vanuatu (New Hebrides) arc; TA, Tongan arc; F, Fiji.

Tasmania (Pyle et al. 1995). To our knowledge, there is no The latter authors have shown that many Poya Terrane information on dredged rocks from the floor of the New basalts have geochemical affinities with backarc basin Caledonia Basin. basalts. We concur with Cluzel et al. (2001) that the Poya Evidence for the nature and age of the earliest oceanic Terrane basalts represent early stage magmatism in the vol- crust along the northwestern margin of the South Loyalty canic passive margin of a backarc basin. We argue that this Basin is found in the Formation Des Basaltes of the basin formed during fragmentation of a Cretaceous arc con- allochthonous Poya Terrane in New Caledonia. Poya structed on thinned continental crust at the leading edge of Terrane rocks contain Campanian to Late Paleocene (ca the Australian Plate. 80–55 Ma) microfossils (Aitchison et al. 1995; Cluzel et al. There is no evidence in the Poya Terrane for extension- 2001) and were emplaced as a major allochthon in the lat- related volcanic rocks as old as the 120 Ma sequences est Eocene (Cluzel 1998; Cluzel et al. 2001) in a south- identified seismically in the offshore South Loyalty Basin westerly direction over continental crust at the leading edge by Auzende et al. (2000). There are several possible rea- of the Australian Plate. Basaltic lavas in the Formation Des sons for this: (i) it is possible that seismic correlations with Basaltes include E-MORB and N-MORB lavas with slight to sequences west of and on New Caledonia are incorrect or significant negative Nb anomalies, and εNd values inapplicable; (ii) the Early to Middle Cretaceous sedimen- between + 3 and +5 (Eissen et al. 1998; Cluzel et al. 2001). tary packages identified seismically in the South Loyalty Tectonic evolution: SW Pacific 381

Basin (Auzende et al. 2000) may be early sag and rift phase South Atlantic, and on the northwestern margin of sequences that significantly pre-date crustal rupture and Australia, was marked by excess basaltic magmatism at the ocean crust accretion, dated by the Poya Terrane basalts as continent–ocean boundary and the production of seaward- occurring around 80–70 Ma; and (iii) microfossils indicat- dipping reflector packages along several thousand kilome- ing a Maastrichtian through Late Eocene age for at least tres of the conjugate passive margins (Planke et al. 2000). some of the Poya Terrane basalts (Cluzel et al. 2001) may Unlike the South Atlantic margin, for example, seaward- be from the sedimentary cappings on post-oceanic crust dipping reflector packages are presently unknown along the accretion intraplate seamounts identified among the Poya Tasman Sea–Lord Howe Rise margin. The western margin Terrane basalt piles. For the present time, we regard accre- of the Lord Howe Rise is a classic lower plate margin as tion of oceanic crust in the South Loyalty Basin as being defined by Lister and Etheridge (1989). It is underlain by a essentially contemporaneous with that in the Tasman Sea. 200 km-wide rift zone characterised by eroded tilt blocks Further evidence for the existence of Cretaceous and half-grabens, with west-dipping faults and eastward- oceanic crust in the South Loyalty Basin east of the New rotated fault blocks. The eastern Lord Howe Rise margin is Caledonia–Norfolk Ridge is also preserved much further an eroded basement block about 150–200 km wide that south, in the Northland allochthon in New Zealand. Here, resembles an unstructured ribbon-type marginal plateau. southwest-directed emplacement of Late Cretaceous Evidence for a volcanic passive margin along the western oceanic basalts onto the North Island occurred around side of the South Loyalty Basin is present as the Poya 25–23 Ma (Malpas et al. 1992; Mortimer et al. 1998; Terrane basaltic allochthon. Instead of a single rift devel- Nicholson et al. 2000). The basalts show a compositional oping into an ocean basin South Atlantic-style, extension in range very similar to that of the Poya Terrane basalts, with the Tasman Sea–Lord Howe region was distributed over a both N-MORB and E-MORB represented. Some basalts much wider area. We suggest that this exceptionally wide show negative Nb anomalies implying, according to zone of rifting along eastern Gondwana developed in Nicholson et al. (2000), generation in a backarc basin close response to rapid rollback of the subducting plate further to an island arc. Whether the Northland allochthon repre- east. Whether this subducting plate was west-dipping sents the terminal phase of a diachronous collision that Pacific Plate, or perhaps east-dipping crust being sub- commenced in the New Caledonia region ca 40–34 Ma and ducted along the eastern side of the New swept south over 20 million years, or whether they are two Caledonia–Norfolk Ridge (Sdrolias et al. 2002) is presently quite unrelated collision events, is presently unknown. uncertain. We believe that excess magmatism due to sud- den decompression at breakup, which might normally be focused into a developing rift to form seaward-dipping A PLUME-TRIGGERED EVENT reflector packages along the margins of a single ocean Evidence summarised above indicates that following a sub- basin, was instead distributed over a large area and led to stantial period (120–80 Ma) of crustal extension and basin near-simultaneous formation of the Tasman Sea and South formation within the continental crust of eastern Australia, Loyalty Basin. Accretion of oceanic crust ceased abruptly in oceanic crust accretion commenced in the Tasman Sea and the Tasman Sea at ca 52 Ma, probably in response to the South Loyalty Basin at about 85 Ma. Why was basin for- first effects on global plate motions of the India–Asia colli- mation and eventual ocean crust accretion in this eastern- sion. most part of Gondwana continental crust so widely distributed, so that instead of a single ocean basin being Formation of microcontinental blocks: the formed, two subparallel marginal seas and the intervening Müller–Gaina model New Caledonia Basin rift developed? Evidence for the arrival of a major mantle plume Gaina et al. (1998) showed that there appear to have been beneath the southern Lord Howe Rise–east Tasmania 13 microcontinental blocks involved in the opening of the region very close to 100 Ma is widespread. Lanyon et al. Tasman Sea. An important aspect of the diachronous peel- (1993), Weaver et al. (1994) and Storey et al. (1999) ing off of the long, compound continental crustal slice to showed that a profound change in the isotopic composition form the Lord Howe Rise (and crust further east on the of intraplate basalts occurred in Tasmania, New Zealand Norfolk Ridge) is that small blocks of continental crust were and the Marie Byrd Land–Transantarctic Mountains region isolated by unsuccessful rifting events. Notably, the East of Antarctica immediately after 100 Ma. All basaltic mag- Tasman Plateau separated from the Lord Howe Rise by 83 matism since about 100 Ma has a distinct HIMU isotopic Ma, the Gilbert Seamount Complex crustal block by ca 77 fingerprint, indicating massive-scale emplacement of Ma, and the Dampier Ridge by 64 Ma. All these small HIMU source mantle beneath this region. No large igneous crustal blocks were formed by ultimately unsuccessful ridge province has been linked to arrival of this plume head propagation and the jumping of the spreading ridge to a beneath the regional southeastern Gondwana lithosphere, more favourable location (Gaina et al. 2002). Based on the although regional uplift is very well documented (Baker & Lord Howe region, and other examples including the Seward 1996; O’Sullivan et al. 2000; Spell et al. 2000). Seychelles, Elan Bank (Kerguelen Plateau) and the The role of mantle plumes in initiating crustal extension Wallaby Plateau off northwestern Australia, Müller et al. and ocean opening has been emphasised by White and (2001) and Gaina et al. (2002) proposed that this ridge- McKenzie (1989), who demonstrated the occurrence of vol- jump model is an important mechanism in the formation of canic passive margins in such plume-triggered rift-ocean marginal continental ribbons. A key feature in this model is settings. Well-known examples include the North and the role of a mantle plume, the existence of which at ca 100 South Atlantic margins. Breakup in both the North and Ma is well established for the southern Tasman Sea–Lord 382 A. J. Crawford, S. Meffre and P. A. Symonds

Howe Rise region. As a young continental margin passes has formed over subducting Pacific crust since ca 45 Ma, close to or over a plume, the inboard flank of the rifted mar- arc magmatism either shut down or diminished remarkably gin is thermally weakened, forcing a ridge jump to this zone in volume during opening of the Parece Vela–Shikoku of weakness and eventual calving-off of a microcontinental backarc basin from 32 to 16 Ma (Crawford et al. 1981). slice. Thus plume–ridge interactions along the eastern The reason for the apparently poorly represented or Gondwana margin around 100 Ma, coupled with a retreat- absent arc-type magmatic products associated with this ing subducting slab, generated the microcontinental rib- Cretaceous subduction is a first-order problem for future bons of the Lord Howe Rise and associated blocks, and the studies of the southwest Pacific sea floor. Norfolk–New Caledonia Ridge. What is the fate of these microcontinental ribbons? To Subduction initiation at ca 45 Ma? investigate this problem, we need to examine the geody- namic evolution post-100 Ma of this outboard section of Numerous previous workers have proposed that subduc- the eastern Gondwana margin, directly facing the Pacific tion initiation around the western Pacific commenced Plate, especially with respect to its history of subduction. around 45 Ma, as a consequence of plate boundary read- justments following collision of India with Asia (Veevers 2000b). In the Mariana–west Philippine region, subduction SUBDUCTION-RELATED TECTONICS initiation generated a nearly 3000 km-long, several hun- dred kilometres-wide belt of new lithosphere dominated by Subduction before 45 Ma boninitic lavas and their cumulate counterparts (Crawford et al. 1981; Stern & Bloomer 1992). However, recent dating We have noted above that the enormous extension of east- of these rocks indicates eruption ages between 52 and 48 ern Australian lithosphere and formation of marginal seas Ma (Cosca et al. 1998). The key point is that subduction between ca 100 Ma and 45 Ma, occurring adjacent to a was perhaps as much as 10 million years before 45 Ma. boundary between the Australian and Pacific Plates, would There are several models accounting for the production appear to demand contemporaneous subduction and slab of this massive belt of boninitic lithosphere (Crawford et al. rollback. A number of workers (Dubois et al. 1973; Collot et 1989; Pearce et al. 1992; McPherson & Hall 2001). The key al. 1987) proposed that east-directed subduction of New ingredient in any model must be that the partial melting of Caledonia Basin oceanic crust occurred before 80 Ma, with previously melted, highly depleted lithospheric mantle the trench occurring along the western edge of the New requires exceptionally hot conditions (>1200°C) at very Caledonia–Norfolk Ridge. However, this model demands shallow levels (<50 km) in the subduction zone. Such con- the presence of a Late Cretaceous–Paleocene island arc in ditions are not normally present in intra-oceanic subduc- the immediate New Caledonia–Norfolk Ridge region for tion zones at depths < 50 km. We prefer a model for which there is still no evidence (Regnier 1988). boninite generation in the Mariana–Bonin system in which Furthermore, there is no convincing evidence that the the extra heat required for boninite production derives from stretched continental crust of the New Caledonia Basin subduction of an active spreading centre orientated almost ever reached the stage of rupturing and accretion of parallel to the trench (Crawford et al. 1989), the ridge oceanic-type crust, so that we consider subduction of New involved being that on the now-subducted North New Caledonia Basin crust to be highly unlikely. Guinea Plate (Seno & Maruyama 1984). Sdrolias et al. (2002) also supported the existence of Eocene boninites are also present as thrust slices Cretaceous subduction east of Australia, and argued for beneath the main New Caledonia ultramafic ophiolite and west-dipping subduction of Pacific oceanic crust prior to in the forearc region of the north Tongan arc (see later). We 120–100 Ma, with arc magmatism occurring on the Norfolk believe that these formed at the same time as the main Ridge. This was followed by east-dipping subduction Mariana–west Philippine boninitic crustal sections, the between 90 and 45 Ma, with the trench lying west of the extra heat required in this model being due to subduction Loyalty–Three Kings Ridge. Again, arc-type volcanics of nucleating along a recently inactive spreading centre in the Late Cretaceous to Eocene age are thus expected along the South Loyalty Basin (Eissen et al. 1998). We argue that Norfolk and Loyalty–Three Kings Ridges, but to date none these boninitic successions represent a key event in the have been found. western Pacific region, recording a major change in plate Based on modern west Pacific arc–backarc basin sys- kinematics at ca 55 Ma, not 45 Ma, and an eventual shift tems, whether above west-directed (e.g. Tonga, Mariana, to more west-directed motion of the Pacific Plate. What is Ryukuyu systems) or east-directed (e.g. Vanuatu arc) sub- the record of this Eocene subduction event in the south- duction, extension is always on the upper plate, in part as west Pacific? a consequence of rollback of the subducting plate. This pre- dicts that the Cretaceous–Paleocene extensional setting Loyalty Ridge–D’Entrecasteaux Ridge east of Australia was due to massive rollback of the Pacific Plate, demanding west-directed subduction of Pacific The eastern margin of the South Loyalty Basin is defined by Oceanic crust (Figure 3a). Perhaps slab rollback was so the Loyalty Ridge, a discontinuous bathymetric high rapid and effective that arc magmatism was dampened or formed by well-spaced seamounts that occasionally emerge suppressed, and the major magmatic expression of subduc- to form the Loyalty Islands. The Loyalty Ridge trends north- tion was backarc basin-type ridge-generated oceanic crust west (Figure 2) before swinging around clockwise to form of the Tasman Sea and South Loyalty Basin. In the the east-trending D’Entrecasteaux Ridge that is presently Mariana–West Philippine arc–backarc basin system that colliding with the mid-segment of the Vanuatu island arc. Tectonic evolution: SW Pacific 383

Between the Loyalty–D’Entrecasteaux Ridge and the active Eissen et al. (1998) proposed a model for the tectonic Vanuatu (New Hebrides) trench is the North Loyalty Basin development of this region in which east-directed subduc- (Weissel et al. 1982). tion initiated at 50–45 Ma along the active spreading cen- Islands and seamounts forming the Loyalty–D’Entre- tre in the South Loyalty Basin (Figure 3b). Abnormally high casteaux Ridge show evidence of at least two stages of mag- mantle temperatures at the initiation of subduction led to matic activity. Intraplate basalts that were erupted between boninite formation in the forearc region of the resultant 11 and 8 Ma cap Mare, one of the southern Loyalty Islands Loyalty arc (Crawford et al. 1989). Continuing east-directed (Monzier et al. 1989). In contrast, dredging and ODP subduction of the western half of the South Loyalty Basin drilling has shown that the seamounts further north, led to growth of the Loyalty–D’Entecasteaux arc and the including one in collision with the Vanuatu arc, are typical eventual arrival of the Late Cretaceous volcanic passive intra-oceanic arc volcanoes, dominated by primitive arc margin of this basin at the trench. West-directed emplace- tholeiitic lavas (Maillet et al. 1983; Baker et al. 1994; ment, first of the volcanic passive margin-derived Poya Coltorti et al. 1994a). ODP drill site 831 on Bougainville Terrane (Figure 3b), followed by the main New Caledonian Guyot, located at the eastern extremity of the forearc-derived ophiolite, occurred around 40–34 Ma D’Entrecasteaux Ridge, has shown that this was an Eocene (Cluzel et al. 2001), terminating subduction and leading to arc volcano. Furthermore, rocks drilled beneath middle subsidence of arc volcanoes such as Bougainville Guyot. Eocene ooze at ODP Site 828 on the North As the age of the youngest magnetic anomaly in the North D’Entrecasteaux Ridge north of Bougainville Guyot are Loyalty Basin identified by Sdrolias et al. (2002) is 35.3 Ma, also primitive arc tholeiitic magmas appropriate for a fore- the cessation of backarc spreading was broadly contempo- arc setting of magma generation (Coltorti et al. 1994b). At raneous with shutdown of the arc. about 38–37 Ma, magmatism terminated and Bougainville The initial phase of collision involved emplacement of Guyot volcano subsided rapidly (Quinn et al. 1994; Baker one or more slices of the Late Cretaceous volcanic passive et al. 1994). margin at the leading edge of the Australian Plate back On this basis, most authors agree that the southwestward over the adjacent thinned continental crust Loyalty–D’Entrecasteaux Ridge is probably an Eocene (Figure 3b). These volcanic-dominated allochthonous island arc formed over an east- and southeast-dipping sub- sheets are now represented by the Poya Terrane, best duction zone (Figure 3b) with a trench originally located exposed along the western coast of New Caledonia. between the present east coast of New Caledonia and the Subsequent thrusting emplaced slices of forearc crust and Loyalty Ridge (Maillet et al. 1983; Aitchison et al. 1995; upper mantle over the Poya allochthon (Eissen et al. 1998; Eissen et al. 1998; Ali & Aitchison 2000). There are no con- Cluzel et al. 2001). The uppermost structural slice domi- straints presently available on the timing of initial arc mag- nates the ophiolite and is made up largely of uppermost matism on the Loyalty–D’Entrecasteaux arc. If the North mantle rocks lacking a crustal section (Figure 3b). It may be Loyalty Basin is regarded as a backarc basin that developed that the crustal section was sliced off as an earlier behind this arc, then magnetic anomalies identified as allochthon (represented, albeit in very small outcrop areas, 56–50 Ma by Weissel et al. (1982) and Collot et al. (1985) by the boninites around Nepoui), to be subsequently over- suggest that the Loyalty arc was active before 56 Ma. ridden by the uppermost mantle section. However, in a new interpretation of these anomalies, Sdrolias et al. (2002) have proposed instead that they were Post-collisional tectonic evolution and exhuma- formed between 43.8 and 35.3 Ma, an age range consistent tion of underthrust continental crust with the North Loyalty Basin being a backarc basin linked to the Loyalty–D’Entrecasteaux arc. Mid-Eocene volcani- Leading up to, and following emplacement of, the main clastic sandstones of arc provenance overlying basaltic New Caledonia ophiolite at 40–34 Ma, a foredeep devel- basement at DSDP site 286 (Andrews et al. 1975) provide oped in which bioclastic and pelagic sediments were over- a minimum age for this northern part of the North Loyalty lain by coarse clastic sediments and olistostromes, Basin crust. contemporaneous with thrust faulting (Aitchison et al. Maillet et al. (1983), Aitchison et al. (1995), Eissen et al. 1995; Cluzel et al. 2001). Igneous and sedimentary units of (1998), Ali and Aitchison (2000) and Cluzel et al. (2001) the volcanic passive margin and adjacent thinned Early have all proposed very similar tectonic models in which the Mesozoic continental crust on the downgoing plate were forearc of the Loyalty arc collided in the late-Early to mid- metamorphosed to eclogite and blueschist facies (Clarke et Eocene (around 46–38 Ma) with the Norfolk Ridge, which al. 1997) in the subduction zone above the overthickened at that time constituted the thinned leading edge of the crust. Exhumation and tectonic denudation of these high- Australian Plate. This led to emplacement of the New pressure rocks (Figure 3b) occurred in response to post-col- Caledonia ophiolite. Seismic transects reported by Collot et lisional extension and development of large detachment al. (1987) show oceanic crust of the South Loyalty Basin to faults (Rawling & Lister 2000). Sediments produced during the west of the Loyalty Ridge rising towards, and being con- this unroofing process and accumulated in basins on either tinuous with, the New Caledonian ophiolite. The highly side of New Caledonia are 1–1.5 s (TWT) thick (Auzende et refractory nature of the mantle section of the New al. 2000). Below them, an erosional break between Middle Caledonian ophiolite (Prinzhofer 1981; Leblanc 1995), and Eocene, and Late Eocene to Late Oligocene strata reflects the occurrence of low-Ca boninites in fault slices beneath emplacement of the ophiolite. the mantle section of the ophiolite (Cameron 1989), There is some evidence that cessation of east-dipping strongly support a forearc origin of the main ophiolite on subduction may have swept southward, affecting New New Caledonia. Caledonia around 40–34 Ma and as late as 25 Ma when the 384 A. J. Crawford, S. Meffre and P. A. Symonds

3a

Figure 3 Model for the tectonic evolution of the southwest Pacific from 70 Ma to present. Heavy arrows show approximate plate motion directions (a) Setting at ca 70 Ma showing ongoing accretion of oceanic crust in the Tasman Sea and South Loyalty Basin, and crustal extension in the New Caledonia Basin, over a west-dipping subduction zone marked by rapid rollback of the trench. By 50 Ma, northward propagation of the Tasman Sea spreading ridge has detached the Dampier Ridge, and spreading ceased at this time in the Tasman Sea and South Loyalty Basin. East-dipping subduction was initiated between 55 and 50 Ma along the recently inactive South Loyalty Basin spreading centre and generation of boninitic lavas in the forearc of the new Loyalty–D’Entercasteaux arc, which is built upon pre-existing arc crust. (b) At 45 Ma, a major change in global plate kinematics leads to initiation of west-dipping subduction, as opening of the North Loyalty Basin starts to unzip the arc. By ca 40 Ma arrival of the western volcanic passive margin of the South Loyalty Basin (eastern margin of the Norfolk–New Caledonia Ridge) at the trench in front of the Loyalty–D’Entrecasteaux arc leads to emplacement of the Poya Terrane nappe, derived from rift tholeiitic basalts of the volcanic passive margin, and subsequent emplace- Tectonic evolution: SW Pacific 385

3b

ment of boninitic lava slices and sub-boninite refractory uppermost mantle of the forearc region of the Loyalty-D’Entrecasteaux arc to form the New Caledonia ophiolite (see cross-section). The arc subsides by 38 Ma. From 35–30 Ma, the South Fiji Basin continues to grow and the Vitiaz arc extends southward. (c). A western spreading centre limb in the South Fiji Basin propagates into the Norfolk Ridge, detaching the Three Kings Ridge and terminating the east-dipping subduction. Spreading in the South Fiji Basin terminates around 25 Ma, and Cretaceous basalts are thrust southwestward over Northland in New Zealand. Following final collision of the Ontong Java Plateau with the Solomons arc and blocking of this plate boundary around 10 Ma, flipping of the subduction zone to east- dipping occurred, with near-contemporaneous splitting of the Vitiaz arc to form the North Fiji Basin. Volcanism on the Vanuatu arc commences around 8–5 Ma and continued rotation of the Fiji and Vanuatu segments of the Vitiaz arcs occurs via further opening of the North Fiji Basin. Rifting of the Tonga–Kermadec arc starts in the north at ca 6–5 Ma with opening of the Lau Basin. 386 A. J. Crawford, S. Meffre and P. A. Symonds

3c

Northland allochthon was emplaced. Whether this tre in the Loyalty Basin generating, in the first instance, diachronous collision commenced further north than the primitive boninitic crust and complementary refractory New Caledonia region is unknown at present. harzburgitic upper mantle. As the subduction system reached steady state, individual arc volcanoes of the Eocene subduction: regional evidence Loyalty–D’Entrecasteaux arc were constructed by low-K arc tholeiitic magmas. However, there is strong evidence, We have argued above that in response to a major change best shown by the bend in the Hawaii–Emperor chain, that in global plate motions at ca 55 Ma, east-dipping subduc- another major phase of plate reorganisation commenced tion commenced along the recently inactive spreading cen- around 45 Ma, with subduction in the southwest Pacific Tectonic evolution: SW Pacific 387 region changing from east-directed to west-directed polar- Tonga and Viti Levu. Volcanic clasts in these conglomerates ity. We believe that a new arc system, the Vitiaz arc, formed are commonly angular and appear to be locally derived above this west-dipping subducted Pacific Ocean crust (Neef et al. 1985). Carney (1985) considered these clasts to (Figure 3b). Evidence reviewed below supports initiation of be sourced from a reef-fringed Upper Eocene to the Vitiaz subduction zone around 45 Ma. Lower–Middle Miocene tholeiitic island arc. New data for the small much-disrupted Basement Tonga–Kermadec arc Complex ophiolite on the island of Pentecost in the Vanuatu arc provide useful information on the composition The oldest known rocks in the Tonga–Kermadec arc are and age of the basement on which the Vanuatu arc devel- 46–40 Ma (Ewart et al. 1977; Duncan et al. 1985) arc-type oped. Uplift and emplacement of the ophiolite into overly- lavas occurring below upper Middle Eocene limestones on ing Miocene volcanic and volcaniclastic rocks have been Eua Island (Tappin & Balance 1994) in the forearc region linked to collision of the D’Entrecasteaux Ridge with the of the Tongan arc. ODP drilling at Site 841 in the Tongan modern Vanuatu arc at 3–2 Ma (Greene et al. 1994). The forearc recovered a thick sequence of low-K arc tholeiitic ophiolitic lavas, dolerites and peridotites have composi- rhyolites (Bloomer et al. 1994), dated by McDougall (1994) tional affinities with backarc basin crust and uppermost at 44 ±2 Ma. Further north in the Tongan forearc, true low- mantle (Crawford et al. 1998). A doleritic dyke from the Ca boninitic rocks and associated backarc basin-type ophiolite has yielded an Ar/Ar plateau age of 44.9 ±3.0 Ma, basalts of probable Eocene age have been dredged at suggesting that the early formed part of the Vanuatu arc depths in excess of 4 km (Falloon et al. 1998) and have may have been originally constructed on Eocene basaltic yielded dates between 45 and 35 Ma (Bloomer et al. 1998). crust of the early North Loyalty Basin. An amphibolitic On available evidence, we cannot be sure whether these mylonite and a greenschist facies metadolerite from the boninitic lavas were generated in the same ca 55 Ma sub- Pentecost ophiolite have Ar/Ar plateau dates of 35.5 ±0.6 duction-initiation event that generated the boninite–refrac- Ma and 33.4 ±0.8 Ma respectively (S. Meffre & A. J. tory-forearc mantle package of the New Caledonia Crawford unpubl. data). These ages are taken to date an ophiolites further north along the same plate boundary, or important episode of regional deformation and recrystalli- whether they represent magmatism associated with the 45 sation, presumably that responsible for the cessation of Ma Vitiaz arc subduction-initiation event. spreading in the North Loyalty Basin at ca 35 Ma.

Fiji region Vitiaz arc In Fiji, the oldest known rocks are those constituting the We believe that following emplacement of the New Yavuna Group of western Viti Levu (Rodda 1994). Volcanic Caledonia ophiolite during arc–microcontinent collision at rocks include common pillow basalts for which very little ca 38 Ma, subduction jumped outward to become west- data are available. Late Eocene ages have been established directed and to form the Vitiaz arc, comprising what is now for overlying limestones (Hathway 1994). Further geo- the older parts of the Tonga–Fiji–Vanuatu and Solomons chemical and dating studies of the Yavuna Group are arc systems (Figure 3b). By 35 Ma, this arc had started to required to compare them with other Eocene magmatic split, forming the South Fiji Basin. Although linear mag- suites in the region. netic anomalies define a complex pattern of spreading, Yavuna Group lavas are intruded by extensive dyke including two triple junctions (Figure 3c), it is relatively well swarms that are dominated by basalts and basaltic established that spreading commenced in the South Fiji andesites, and subordinate felsic dykes, with affinities tran- Basin in the Early Oligocene around 32 Ma and ceased sitional between arc tholeiites and backarc-basin basalts about 25 Ma (Weissel 1981; Davey 1982; Malahoff et al. (Wharton et al. 1995), and some lavas transitional to boni- 1982; Sdrolias et al. 2002). Cessation of spreading at about nite compositions (Gill 1987). These in turn are intruded by 25 Ma coincides with a major reorganisation of plate the tonalitic-trondhjemitic Yavuna stock, dated at 34.9 ±0.9 boundaries in the western Pacific region at this time (Yan & Ma by K/Ar on hornblende (Rodda 1994) and 36.6 Ma by Kroenke 1993; Sdrolias et al. 2002). However, continuing U–Pb zircon dating (I. Williams pers. comm. in Wharton et subduction along the Vitiaz arc post-25 Ma (Figure 3c) is al. 1995). well supported by numerous radiometric dates on lavas in at least the Vanuatu and Fijian sections of the Vitiaz arc Vanuatu arc (Greene et al. 1994; Whelan et al. 1985; authors unpubl. data). There is no record at present of Eocene lavas in the Initial collision of the Ontong Java Plateau large igneous Vanuatu arc, the oldest rocks being arc-type lavas on the province with the Solomons section of the Vitiaz arc may Western Belt islands of Santo, Malakula and the Torres have occurred as early as 25–20 Ma (Kroenke 1984; Yan & Group. These largely basaltic and andesitic lavas are exten- Kroenke 1993; Petterson et al. 1997). However, the main sively intruded by basaltic to basaltic andesite dyke swarms episode of blocking and compression relating to this colli- and by the Navaka Gabbro, which is Ar/Ar dated at 23 Ma sion was probably between 10 and 5 Ma (Petterson et al. (S. Meffre & A. J. Crawford unpubl. data). On the island of 1997; Wessel & Kroenke 2000). This led to a flip in sub- Maewo, Carney and Macfarlane (1978) reported Lower duction polarity at about 10 Ma, and shortly thereafter (8–7 Miocene conglomerates that contain reworked Eocene Ma) to opening of the North Fiji Basin backarc basin limestones and undated volcanics that show low-K arc (Figure 3c). The distribution and orientation of spreading tholeiitic affinities which match the Eocene volcanics in centres in the North Fiji Basin has been remarkably tran- 388 A. J. Crawford, S. Meffre and P. A. Symonds sient (Auzende et al. 1988) with numerous microplates, Summary parallel spreading ridges, several unstable triple junctions, As a result of regional extension along the eastern margin of and probably leaky transform faults. During opening of the Gondwana since ca 120 Ma, several microcontinental rib- North Fiji Basin, Fiji has rotated anticlockwise by more bons were calved off Australia and were separated by inter- than 100° away from a position on the Vitiaz arc originally vening marginal ocean basins. This is presumed to have along strike from the Vanuatu and Tongan arcs. The occurred over a slab of Pacific oceanic crust which was Vanuatu section of the Vitiaz arc, in contrast, has rotated rapidly rolling back eastward. A major reorganisation of plate clockwise (Musgrave & Firth 1999), creating the North Fiji boundaries at ca 55 Ma, probably induced by India–Asia col- Basin in the manner of opening double gates (Falvey 1975, lision, led to the initiation of subduction along former spread- 1978; Taylor et al. 2000). The Hunter Ridge transform, link- ing centres in the easternmost of these marginal basins and ing Fiji and the southernmost part of the Vanuatu arc along the generation of boninite-dominated lithosphere along the the southern margin of the North Fiji Basin, constituted a new convergent plate boundary. Further subduction led to short-lived subduction zone between ca 7 and 3 Ma as construction of the Loyalty–D’Entrecasteaux arc behind the rotation of Fiji forced subduction of crust in the northeast boninitic forearc terrane. Arrival at the trench of the volcanic South Fiji Basin (Figure 3c). Rocks dredged from the embry- passive margin of the Norfolk Ridge microcontinental ribbon onic Hunter Ridge arc include boninitic and arc tholeiitic led to limited underthrusting of the passive margin crust, and lavas (Crawford & Verbeeten 2000). East-directed subduc- consequent westward emplacement of several allochthonous tion beneath the Vanuatu arc has probably been active sheets around 38 Ma. The first and lowest of these sheets rep- since the subduction zone flipped at about 10–8 Ma, resents basaltic lavas and associated rocks of the underthrust although no reliable radiometric dates on lavas in the volcanic passive margin. A second sheet, very poorly active arc are >5 Ma (Macfarlane et al. 1988). exposed, consists of one or more slices of boninitic lavas rep- The Lau Basin–Havre Trough is a well-defined backarc resenting the subduction-initiation forearc package. The high- system (Figure 2) that has split the length of the Miocene est and thickest slice is dominated by the uppermost mantle Lau–Colville Ridge section of the former Vitiaz arc. section that underlay the boninitic forearc lavas sequences. Subduction and consequent backarc-basin opening is Following crustal thickening associated with emplacement of fastest in the north, where a complex triple junction exists; these elements, an extensional regime developed that culmi- in the south, major extension of arc crust has occurred, but nated in exhumation and denudation of the underthrust con- steady-state accretion of oceanic crust may not have com- tinental crust, some of which shows eclogite and blueschist menced. Rifting of the Lau–Colville Ridge is considered to facies assemblages. Locking of this plate boundary, and a have started around 6 Ma (Clift & Dixon 1994). change of spreading direction of the Pacific Plate around 45 Ma, led to outward stepping of the subduction zone to the New Zealand region position of the Vitiaz arc and a new cycle of arc growth, arc splitting and backarc-basin formation. Collision and ophiolite emplacement occurred in the New This ca 38 Ma western Pacific-style arc–microcontinent Caledonia region between 40 and 34 Ma. More than 2000 collision produced a narrow orogen, probably less than 100 km further south, similar Cretaceous oceanic crust was km across. Post-collision magmatism is not known in the thrust westward on to Northland in New Zealand, forming New Caledonia–Loyalty Basin region and probably did not the extensive Northland Allochthon (Malpas et al. 1992; form because the subduction zone stepped Pacific-ward Nicholson et al. 2000) with the date of emplacement being several hundred kilometres. In contrast, more than 1000 25–23 Ma. km further south a collisional event at 25 Ma emplaced Mortimer and Herzer (2000) reported sillimanite–garnet similar Cretaceous oceanic crust onto Northland. Although schists from Cavalli Seamount, immediately northeast of poorly understood at present, extension tectonism in the the tip of Northland and noted that Ar/Ar data suggest these offshore Northland region following the collision led to the derive from a Paleocene protolith rapidly exhumed at generation of high-K calc-alkaline and shoshonitic magmas 25–20 Ma. Mortimer et al. (1998) also reported a 26 Ma between ca 23 and 18 Ma, and these formed seamounts in ‘biotite-rich’ volcanic breccia dredged from the western the southern section of the South Fiji Basin and on slope of the Three Kings Ridge, and far more widespread Northland. Early Miocene (22–18 Ma) potassic lavas, including shoshonites, in a broad region of the sea floor north of Northland and along the eastern side of the Three Kings LATE NEOPROTEROZOIC AND EARLY Rise. The calc-alkaline to shoshonitic nature of these Early PALAEOZOIC SOUTHEASTERN AUSTRALIA Miocene volcanics in Northland, on the submarine Northland Plateau, and on the Three Kings Rise, has been 600–570 Ma mafic volcanic rocks: a volcanic linked to contemporaneous west-dipping subduction by passive margin Mortimer et al. (1998) and Mortimer and Herzer (2000). In contrast, we suggest that these lavas represent either mag- The geological history of southeastern Australia between the matism associated with the final phase of east-dipping sub- Late Neoproterozoic (ca 600 Ma) and the Early Ordovician duction which had been progressively shutting down from (ca 490 Ma) records a cycle of continental rifting and ocean north to south (Figure 3c), or post-collisional magmatism opening (ca 600Ma), subduction (starting ca 515 Ma), and forming seamounts in a regional extensional setting follow- arccontinent collision (Crawford & Berry 1992), with impor- ing emplacement of the Northland allochthon and rapid tant post-collisional extension, magmatism (500 Ma), rollback of the subducting slab. exhumation of underthrust Neoproterozoic continental Tectonic evolution: SW Pacific 389

Figure 4 (a) Regional magnetic image of southeastern Australia, showing prominent linear anomalies in western New South Wales, western Victoria, eastern South Australia and in Bass Strait (from VandenBerg et al. 2000). (b) Map of the northwestern margin of Western Australia at the same scale as (a) showing orthogonal nature of the breakup margin and distribution of seaward-dipping reflec- tor packages and breakup volcanics (modified from Direen & Crawford 2002). crust, and molasse deposition. The dimensions, orientation episode of crustal extension and rift magmatism that prob- and duration of these events bear a striking similarity to ably terminated before rupturing of the continental crust events east of Australia from 100 Ma until the present, and (Crawford & Berry 1992). The axis of extension and mag- argue for very similar processes having affected the eastern matism is presumed to have jumped eastward and been margin of Australia from 600 Ma until the present. successful in opening an ocean basin at 600–570 Ma and Continental breakup and opening of the palaeo-Pacific in doing so, rifting off at least one continental block or has been well defined as occurring between 600 and 560 microcontinental ribbon (Crawford & Berry 1992; Crawford Ma (Crawford et al. 1997; Veevers et al. 1997; Foden et al. & Direen 1998). The presence of high-temperature picritic 2001). Abundant evidence occurs in westernmost New lavas suggests volcanic passive margin development being South Wales and Victoria, as well as on King Island and in triggered by a mantle plume (White & McKenzie 1989). We western Tasmania, for the existence of a 600 Ma volcanic note the proposition put by Williams and Gostin (2000) passive margin along this section of eastern Gondwana. that the remarkable submarine canyons cut into Rift tholeiitic pillow basalts up to 4 km thick occur along Neoproterozoic strata in the Adelaide Rift Complex may be the Koonenberry Belt in western New South Wales due to regional uplift accompanying emplacement of a (Crawford et al. 1997; Crawford & Direen 1998; Direen mantle plume beneath this region post-590 Ma but before 1999), and picritic lavas sequences in which liquid compo- the Precambrian–Cambrian boundary (ca 544 Ma). sitions had >15% MgO are known from King Island In this developing Late Neoproterozoic rift, the orienta- (Waldron & Brown 1993; A. J. Crawford unpubl. data) and tion of spreading centre segments is believed to have been western Victoria (Crawford 1997; Direen 1999), and are northwest–southeast (Figure 4a), in keeping with the cur- associated with magnetic anomalies up to several hundred rent distribution of major rift volcanic piles on the east-fac- kilometres long (Figure 4a). Carbonate sequences dated ing volcanic passive margin formed in this rifting event. The around 600 Ma (Calver & Walter 2000) occur below and northeast–southwest-trending southern margin of the above the rift tholeiite package in the Smithton Trough in Willyama Block is therefore taken to be a former transform northwestern Tasmania, and at this location record an margin at breakup. A second breakup transform may have 390 A. J. Crawford, S. Meffre and P. A. Symonds

Figure 5 Figure 5: Hypothetical tectonic development of the Tasmanian section of southeastern Australia shown as crustal cross-sec- tions between 600 and 480 Ma. (a) ca 600 Ma: plume-triggered rifting at ca 600 Ma produces an east-facing volcanic margin with thick seaward-dipping reflector packages (SDRS). (b) ca 520–515 Ma: east-dipping subduction commences, and produces a boninitic forearc lithospheric section and a primitive intra-oceanic arc. (c) ca 510 Ma: arc–continent collision leads to emplacement of allochthons first of SDRS-type volcanic passive-margin basalts, which are overridden by nappes of forearc-derived boninitic lithospheric sections, lead- ing to collapse of the margin and development of the Dundas Trough foreland basin. (d) 505 Ma: the new crustal collage had com- menced to collapse due to post-collisional extension, and Mt Read Volcanics post-collisional lavas were erupted in the half-graben formed between ca 505 and 497 Ma. (e) ca 495 Ma: continued extension led to exhumation of the underthrust continental crust of the volcanic passive margin, and the production of large amounts of coarse proximal siliciclastic molasse (Owen Conglomerate) that filled grabens formed along the collision zone. Tectonic evolution: SW Pacific 391 been located between western Victoria and Tasmania to Victoria and are well exposed in the Heathcote (Crawford account for the eastward shift in the magnetic trends, cor- & Cameron 1985) and Mt Wellington greenstone belts responding to major Neoproterozoic basaltic piles, going (Figure 6), particularly at Howqua (Crawford 1982). from western Victoria to King Island and Tasmania. The Following west-directed emplacement in a Middle scale and orthogonal nature of the Neoproterozoic volcanic Cambrian arc–continent collision, these greenstones now passive margin match well the volcanic passive margin of form the lowest part of major west-dipping listric thrust Western Australia that developed between ca 155 and 125 duplexes emplaced during east-vergent Late Ordovician or Ma (Figure 4b). Early Silurian deformation (Fergusson et al. 1986; Gray & Foster 1997). Boninitic lavas occur at the base of the lava Subduction, ocean closing and post-collisional piles, are overlain by black shales, and followed by up to tectono-magmatic events 1.5 km thickness of backarc basin-type tholeiitic pillow basalts. Conformably overlying the basalts are occasional Dating the initiation of subduction in this new ocean basin cherts and a thick pile of Ordovician quartz-rich turbidites is difficult. Boninitic lava –cumulate piles in western that extend across most of the Lachlan Fold Belt from west- Tasmania and Victoria have yielded Middle Cambrian ern Victoria to east of Canberra. The Moyston Fault just ages (515–510 Ma: Turner et al. 1998). These are consid- east of the Grampians in western Victoria is believed to ered to have been generated by subduction of a spreading delineate the western edge of the Lachlan Fold Belt and its centre, or subduction initiation at a spreading centre, and contact with the Delamerian Fold Belt (VandenBerg et al. to have formed the forearc region of an island arc (Figure 2000). 5a, b) that was constructed by ongoing subduction Post-collisional calc-alkaline volcanics with U–Pb zir- (Crawford & Berry 1992). Collision at 510–505 Ma con dates close to 500 Ma and akin to the Mt Read between the 600 Ma east-facing volcanic passive margin Volcanics occur in the Stavely greenstone belt in western and the forearc region of this intra-oceanic arc led to west- Victoria (Figure 6) and are best known from the Mt Stavely directed emplacement of one or more extensive Volcanic Complex (Crawford et al. 1996; Crawford et al. in allochthons of forearc-derived boninites and associated press), which outcrops south of the Grampians and con- low-Ti tholeiitic basalts on to the volcanic passive margin sists of fault-bounded but internally undeformed blocks of (Figure 5c). At almost every location where the contact is volcanic and volcaniclastic rocks. Detrital zircons in a vol- exposed in western Tasmania the ophiolitic rocks are caniclastic rock and magmatic zircons in a dacite of the Mt thrust over rift tholeiitic basalts of the Crimson Creek Stavely Volcanic Complex have yielded crystallisation ages Formation and the 600–570 Ma volcanic passive margin of 501 ±9 Ma and 495 ±5 Ma respectively (Stuart-Smith & sequences. Faulted contacts with older units, and signifi- Black 1999). Cambrian calc-alkaline volcanics with cant changes in the geochemistry of these basalts at differ- medium- to high-K affinities also occur as thrust slices ent locations (Brown & Jenner 1989; A. V. Brown & J. L. along the eastern margin of the Melbourne Zone at Everard pers. comm. 2000), suggest that the rift tholeiitic Jamieson and Licola (Figure 6). In terms of age and geo- package may also represent one or more allochthonous chemical composition, these lavas are strikingly similar to sheets that were emplaced just before, and were overrid- the Mt Read Volcanics of western Tasmania (Crawford et al. den by, the ophiolitic allochthon (Figure 5d). The relative in press) and they have been interpreted to represent ero- orientation and geological results of this collision are sion windows through to an older basement of Tasmanian remarkably similar to those that occurred almost 500 mil- affinity, named the Selwyn Block (VandenBerg et al. 2000). lion years later during the collision of the Loyalty forearc The Selwyn Block is interpreted to extend from with the volcanic passive margin of the eastern side of the Tasmania northward under Bass Strait and to occur under Norfolk Ridge. In both cases, the orogen formed by the col- most of the Melbourne Zone in Victoria (VandenBerg et al. lision was probably less than 100 km wide. 2000). VandenBerg et al. (2000) showed the Selwyn Block Extension of the newly assembled Middle Cambrian as a microcontinental block probably some 150–200 km crustal collage led to formation of half-grabens into which, wide, and lying 400–500 km east of the eastern edge of the in Tasmania, the Mt Read Volcanics were erupted around Delamerian Fold Belt in the Late Cambrian, separated from 500 Ma almost entirely in a submarine setting (Figure 5d). it by Cambrian (backarc basin) oceanic crust represented Further extension in the Late Cambrian led to gradual by the Cambrian tholeiites in the Heathcote and Mt exhumation of passive-margin crust, some of it bearing Wellington greenstone belts. By implication, the Selwyn eclogite–whiteschist assemblages, from beneath overthrust Block should be the microcontinental block rifted from cra- boninitic allochthons (Crawford & Berry 1992; Meffre et al. tonic Australia during the ca 600 Ma rifting event. 2000), so that outcrop of the Mt Read Volcanics was effec- Following the Delamerian deformation in the Late tively limited to the western and northern margin of Cambrian, the subduction zone is believed to have jumped exhumed crystalline crust (Figure 5e). Rapid erosion of the outboard, east of the Selwyn Block microcontinent, to form actively emergent crystalline crust led to deposition of the Early Ordovician to Late Ordovician Macquarie Arc coarse proximal siliciclastic molasse (Owen Conglomerate (Glen et al. 1998). Arc-type lavas as old as 480 Ma are and correlates), in part unconformably above the Mt Read recorded from the Ordovician volcanic Parkes–Narromine Volcanics. belt (Butera et al. 2001), which trends southward into Direct correlates of the western Tasmanian boninitic northeastern Victoria around the headwaters of the Murray allochthons and post-collisional Mt Read Volcanics are River. A detailed discussion of the subsequent Siluro- known across Bass Strait in Victoria. Boninitic lavas of Devonian tectonic development of the Lachlan Fold Belt is Cambrian age occur in all three major greenstone belts in beyond the scope of this paper. However, key features 392 A. J. Crawford, S. Meffre and P. A. Symonds

Figure 6 Map of Victoria showing distribution of the Cambrian and Late Neoproterozoic greenstones across the State, divided into Late Neoproterozoic rift-related basalts, allochthonous tholeiite–boninite packages of Early to early-Middle Cambrian age, and late- Middle Cambrian (ca 500 Ma) post-collisional calc-alkaline (mainly andesitic) lavas (modified from VandenBerg et al. 2000).

worth noting include the development in the Early Silurian which the Lord Howe Rise and Norfolk–New Caledonia through Middle Devonian of subparallel elongate troughs Ridge were detached from eastern Australia between 120 within the Macquarie Arc, including the Cowra and Hill and 80 Ma. End Troughs, and the closing and deformation of these troughs during the Middle and Late Devonian New England Fold Belt (Tabberabberan Orogeny). Crust beneath the Snowy Mountains south of Canberra is up to 52 km thick A similar sequence of convergent margin development and (Finlayson et al. 1980), and suggests that a collision–accre- episodic margin fragmentation and accretion of arcs to east- tion event, involving the extended Macquarie Arc and ern Australia occurred between the Devonian and Late another microcontinental block, may have been responsi- Mesozoic with resulting development of the New England ble for the Tabberabberan deformation. Fold Belt (Murray 1987; Aitchison et al. 1994; Holcombe et Collins (in press) has drawn attention to the intermit- al. 1997; Murray et al. in press). Included are continental tently extensional nature of the Lachlan Fold Belt, which margin arc terranes that presumably formed upon the out- he referred to as an extensional accretionary orogen. He board margin of the recently cratonised Lachlan Fold Belt, argued that such orogens initiate at intra-oceanic conver- and tectono-stratigraphic domains more typical of intra- gent margins and grow during slab rollback, by magmatic oceanic arc systems, such as the Gympie ‘terrane’. Calc- and protracted turbiditic sediment additions, in the alkaline magmatism of Late Devonian–Early Carboniferous backarc region and in the arc and accretionary prism. age extends up to 400 km inboard of the convergent plate Evidence of large scale ‘terrane accretion’ is lacking and margin, implying a low angle of subduction beneath the Collins (in press) attributes transient localised compres- continental margin arc at that time. In a recent study of the sional events to the arrival and temporary blocking Gympie Terrane, Sivell and McCulloch (2001) suggested effects of ‘oceanic plateaus’. We agree with many aspects that it formed as a primitive island arc, ‘possibly within pre- of Collins’ model, but argue that for the Tasman Fold Belt existing New England Orogen accretionary complex ele- System, the ‘oceanic plateaus’ are in fact microcontinen- ments’, during Early Permian extension accompanying tal ribbons calved from cratonic Australia probably dur- eastward trench rollback and steepening of the Benioff zone. ing 600–570 Ma rifting, in similar manner to the way in The Sydney–Gunnedah–Bowen Basin system was also initi- Tectonic evolution: SW Pacific 393 ated in the Early Permian by a major extensional event that The Lachlan Fold Belt and New England Fold Belt in affected the New England Fold Belt. From the mid-Permian southeast Australia can be interpreted in the above frame- onwards, most of the New England Fold Belt was a major work, implying that the processes active in the southwest foreland thrust belt that developed in a backarc setting Pacific from 120 Ma have apparently been happening behind the convergent plate boundary and west-dipping along this margin of the Pacific Ocean since at least 600 subduction zone that had migrated to the east (Korsch & Ma. Apart from the immense Ontong Java Plateau colliding Totterdell 1995). Much of the major Mid to Late Triassic with the Solomons since ca 10 Ma, there is no convincing deformation was ascribed by Harrington and Korsch (1985) evidence that any component of either fold belt originated to the arrival and docking of the Gympie Terrane with the very far from the convergent margin east of Australia. Most New England Fold Belt, although Sivell and McCulloch or all of the terranes recognised by previous workers in the (2001) have suggested that accretion of this arc to the con- Lachlan and New England Fold Belts are probably ‘native’ tinental margin occurred in the Early to Middle Triassic. terranes to the region, having developed within or adjacent Despite ongoing uncertainty relating to the tectonic set- to the eastern margin of Australia. ting of a number of key elements of the New England Fold It has been argued that eastern Australia–Antarctica Belt (Bryan et al. 2001; Murray et al. in press), there is lit- was juxtaposed against Laurentia prior to ca 750 Ma, tle doubt that this orogen developed over a period of more together constituting the supercontinent Rodinia (Dalziel than 150 million years by fragmentation of the continental 1991; Hoffman 1991; Moores 1991; Powell et al. 1994). margin during repeated extensional events, growth of new Precisely which part of Laurentia lay against eastern island arcs with accretionary prisms outboard of this mar- Australia is uncertain with candidates including western gin, and their eventual return and accretion on to the con- Canada (Moores 1991), western USA (Burrett & Berry tinental margin. In similar fashion, fragments of this orogen 2000), Mexico (Wingate et al 2001) and South China (Li et have been split off Australia during Tasman Sea opening al. 1995). Powell et al. (1993, 1994) and Powell (1998) sug- and are now located on the Norfolk Ridge (Black 1996; gested that breakup between Laurentia and Meffre et al. 1996), the Lord Howe Rise, and in New Australia–Antarctica happened shortly after 750 Ma and Zealand (Bradshaw 1989; Cawood 1984). that it occurred along the Tasman Line, and Preiss (2000) considered that breakup occurred at ca 700 Ma, closely associated with the Sturtian glaciation. Wingate and CONCLUSIONS AND IMPLICATIONS FOR Giddings (2000) presented evidence indicating that RODINIA BREAKUP breakup occurred prior to 755 Ma. However, we are confi- dent that all available geological evidence indicates that the A review of the tectonic development of the southwest Pacific Tasman Line reflects a disrupted and reworked 600–570 east of Australia from 120 Ma until the present has shown Ma breakup margin (Crawford & Direen 1998). If there was that the products of tectono-magmatic processes occurring in a breakup event some 100-150 million years earlier along this region are remarkably similar to rocks making up the this same margin, the magmatic evidence of this breakup is 600–350 Ma southern section of the Tasman Fold Belt dismayingly small, and evidence of the breakup has been System. This implies that very similar processes operated reworked or overprinted by the later breakup event. from 600 Ma until now to produce these rocks and assemble Strongly influencing thinking regarding the postulated the fold belts. Key processes include: (i) the calving-off of 750–700 Ma age for breakup is the evidence for regional elongate microcontinental ribbons probably induced by one downwarping, intermittent basaltic magmatism and dis- or more mantle plumes; (ii) the initiation of subduction and crete episodes of rifting to form the Adelaide Rift Complex generation of boninitic magmas, probably at a former spread- (Preiss 2000). Early extension is dated at 827 ±7 Ma by the ing centre, as a result of a major change in global plate kine- northwest-trending Gairdner dyke swarm and the matics and reorganisation of plate boundaries; (iii) rapid Wooltana and Beda Volcanics. Later rifting (ca 700 Ma) is passage from boninitic magmatism in an extensional setting, hypothesised to have shifted eastwards and has been inter- to backarc-basin spreading, and eventual growth of an intra- preted as sag-phase deposition associated with continental oceanic arc over a steeply dipping and rolling-back slab; (iv) separation (Preiss 2000). There is no magmatic expression collision of the boninitic forearc of this new arc with a vol- of this hypothetical breakup. canic passive margin of a marginal ocean basin produced We suggest that while palaeomagnetic evidence is during calving-off of the microcontinental fragment(s); (v) inconclusive, and geological tests of Australia–Laurentia emplacement of allochthonous slices, first of volcanic pas- connections remain arguable, it is worth considering sive-margin basalts, then of forearc-derived boninitic rocks another possibility. That is, that eastern Australia (includ- and complementary uppermost mantle, back over the pas- ing its outboard elements such as the Lord Howe Rise) has sive margin crust; (vi) locking of the plate boundary and faced the palaeo-Pacific Ocean since at least 600 Ma and extension-related exhumation of the underthrust volcanic was never a conjugate continental margin (i.e. joined) to passive-margin elements, with high-pressure metamorphic some element of Laurentia or South China. assemblages; (vii) jumping of subduction, probably with a polarity reversal, outboard towards the Pacific and the cre- ation of a new island arc; and (viii) in response to ongoing ACKNOWLEDGEMENTS trench rollback, cycles of arc splitting, backarc-basin open- ing, and the construction of new arcs on the remnants of the A. J. Crawford thanks the late Chris Powell and Keith Crook former arc or on new backarc-basin crust, as exemplified by for directing his focus towards the western Pacific for the North Fiji Basin and Lau Basin. answers to fold belt problems. Dietmar Müller, Maria 394 A. J. Crawford, S. Meffre and P. A. Symonds

Sdrolias and Carmen Gaina are thanked for significant W. & PARIANOS J. 1997. Early Cretaceous volcano-sedimentary input into this paper. Discussions about western Pacific successions along the eastern Australian continental margin: implications for break-up of eastern Gondwana. Earth and matters with Dominique Cluzel, Jean-Philippe Eissen, Planetary Science Letters 153, 85–102. Michele Monzier, Patrick Maillet, Bernard Pelletier, Nick BRYAN S. E., HOLCOMBE R. J. & FIELDING C. R. 2001. Yarrol terrane of Mortimer and Dick Price have been valuable. We acknowl- the northern New England Fold Belt: forearc or backarc? edge numerous useful discussions on Tasman Fold Belt Australian Journal of Earth Sciences 48, 293–316. evolution with Bill Collins, Ron Berry, Nick Direen, Chris BURRETT C. F. & BERRY R. F. 2000. Proterozoic Australia–Western United States (AUSWUS) fit between Laurentia and Australia. Fergusson, David Taylor, Ross Cayley, David Moore, Dick Geology 28, 103–106. Glen, Barney Stevens and Wolfgang Preiss. Geoscience BUTERA K. M., WILLIAMS I. S., BLEVIN P. L. & SIMPSON C. J. 2001. Zircon Australia and the Geological Survey of Victoria are thanked U–Pb dating of Early Palaeozoic monzonitic intrusives from the for permission to use and modify material in Figures 4 and Goonumbla area, New South Wales. Australian Journal of Earth Sciences 48, 457–464. 6 of this paper respectively. Phil Symonds publishes with CALVER C. R. & WALTER M. R. 2000. The Late Neoproterozoic Grassy the permission of the Chief Executive Officer, Geoscience Group of King Island, Tasmania: correlation and palaeogeo- Australia. A. J. Crawford’s Western Pacific research has graphic significance. Precambrian Research 100, 299–312. been funded through Australian Research Council Large CAMERON W. E. 1989. Contrasting boninite–tholeiite associations from New Caledonia. In: Crawford A. J. ed. Boninites and Related Rocks, Grants. 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